9 Major Groups of Invertebrate Animals

There are 30 phyla in the animal kingdom that include invertebrates. This article takes a closer look at the nine most important of these groups. | Source

What Are Invertebrates?

In contrast to bony vertebrates, invertebrates are animals with no vertebral column or backbone. Invertebrates comprise approximately 97% of the animal kingdom. Most of them are soft-bodied and do not develop rigid internal skeletons. Many invertebrates do, however, possess hard exoskeletons to protect their bodies from their environments. Some common examples of invertebrates are snails, sponges, earthworms, squids, sea-stars, centipedes, butterflies, spiders, and jellyfish.

Photos of Some Common Invertebrates

Click thumbnail to view full-size

Unidentified crab from Pexels | Source

Unidentified grasshopper from Pixabay | Source

Unidentified butterfly from Pexels | Source

Unidentified wasp from Pexels | Source

Unidentified jellyfish from Pexels | Source

Unidentified inchworm from Pexels | Source

Unidentified starfish from Pexels | Source

Unidentified anemone from Pexels | Source

The 9 Main Phyla of Invertebrates

Groups of Invertebrate Animals

Examples

Estimated Number of Species

Phylum Porifera

Sponges

3,000

Phylum Cnidarians

Jellyfish, corals, anemones, hydra

9,200

Phylum Platyhelminthes

Flatworms, flukes, tapeworms

15,000

Phylum Echinodermata

Sea stars, brittle stars, sea urchins, sea cucumbers

5,000

Phylum Mollusca

Snails, clams, squids, octopi, other mollusks

70,000

Phylum Nematoda

Ascaris, vinegar eels, hookworms, nematodes, pinworms

500,000

Phylum Arthropoda

Crabs, scorpions, insects, spiders, millipedes, centipedes, barnacles

750,000

Phylum Annelida

Earthworms, leeches, lugworms

22,000

Phylum Chordata

Sea squirts, lancelets

2,000

Click thumbnail to view full-size

Polymastia boletiformis | Source

Unidentified Porifera photo from Wikipedia | Source

Clathrina clathrus | Source

A variety of Porifera | Source

Unidentified Porifera photo from Wikipedia | Source

"Pipe organ" sponges | Source

Unidentified Porifera photo from Wikipedia | Source

1. Phylum Porifera (Sponges)

Sponges, or phylum Porifera, are one of the most common types of invertebrate animals. Currently, there are about 3,000 documented sponge species. The phylum name comes from the Latin words porus, which means "pore," and ferre which means "to bear." the phylum is so named because most sponges bear holes.

Porifera are multicellular animals that consist primarily of tissue and lack complex organs. Most reside in the ocean anchored to coral reefs, rocks, or shells. Sponges occasionally grow on oysters, cover their shells, and prevent them from feeding. This can result in the deaths of the affected oysters and may have an impact on the oyster industry.

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Pelagia noctiluca | Source

Hydra vulgaris | Source

Mushroom corals | Source

Actinia fragacea | Source

Dendronephthya sp. | Source

Entacmaea quadricolor | Source

2. Phylum Cnidaria (Coelenterates)

Cnidarians or Coelenterates are jelly-like aquatic invertebrate animals that are radially symmetrical and have tentacles that encircle a mouth at one end of the body. The life-cycle of a cnidarian includes multiple distinct stages. During the sessile stage, cnidarians resemble cylindrical polyps, while during the medusa stage, they are free-swimming and look like jellyfish.

The name Cnidaria is derived from the Greek word knide, which means "nettle" or "stinging." The word refers to specialized stinging cells called nematocysts that can be projected by certain cnidocytes as a defense. Nematocysts contain poison that can paralyze and sometimes kill other animals. Other examples of cnidarians include corals, hydra, and sea anemones.

Examples of Cnidarians (Coelenterates)

Jellyfish differ in shape, size, and color. Some have shorter tentacles, some have longer tentacles, and some have limbs with nematocysts.

Hydra are small, freshwater organisms that resemble jellyfish. Interestingly, hydra have regenerative properties and don't seem to "age" in a traditional sense.

Corals live in large colonies that serve as sanctuaries and nurseries for fish and other marine organisms.

Click thumbnail to view full-size

Photo of an unidentified marine flatwork from Wikipedia | Source

Photo of unidentified NZ flatworm from Wikipedia | Source

Caenoplana coerulea | Source

Egg of Fasciola hepatica | Source

Helicometria sp. | Source

Taenia saginata | Source

Protoscolex of Echinococcus granulosus | Source

3. Phylum Platyhelminthes (Flatworms)

Platyhelminthes are structurally simple, wormlike invertebrate animals that lack anuses and circulatory systems. The name Platyhelminthes derives from the Greek terms platys, which means "flat," and helmin, which means "worm." There are about 15,000 known species of flatworm. Flatworms' bodies are flattened from the back, or dorsal side, to the belly, or ventral side.

Examples of Platyhelminthes (Flatworms)

Planariamaculata is a free-living invertebrate that occupies freshwater habitats. Adult Planaria are usually about one centimeter in length but can be longer.

Flukes are parasitic worms that live as ectoparasites or endoparasites in various parts of their hosts' bodies, including the intestines, blood, and liver. Flukes have a very simple digestive cavity.

Tapeworms are endoparasites that usually live in the organs of their hosts. They feed on their hosts' digested food but do not have digestive cavities of their own.

Click thumbnail to view full-size

Protoreaster linckii | Source

Acanthaster brevispinus | Source

Gilchrist's sea urchin | Source

Unidentified photo of a sea urchin and its eggs from Wikipedia | Source

Brown-striped brittle stars | Source

Astroporpa annulata | Source

Unidentified photo of sea cucumbers from Wikipedia | Source

4. Phylum Echinodermata (Echinoderms)

Phylum Echinodermata is a group of invertebrate marine animals that have spiny skins. The word Echinodermata comes from the Greek terms echinos, which means "hedgehog," and derma, which means "skin." There are about 5000 species of echinoderms in the world. Members of this group display radial symmetry, and most individuals' bodies are divided into five equal parts that encircle a central axis.

Examples of Echinodermata (Echinoderms)

Starfish inhabit the shallow waters near ocean shores. They come in a variety of colors, shapes, and sizes. There are blue, bright red, reddish-brown, and flesh-colored starfish. They usually have five arms, but some have more than ten. Their arms encircle a small central disk. Some have no visible arms at all and are shaped like balls.

Sea-urchins are the spiniest of all echinoderms. There are several kinds of sea-urchins. The most common are black and have short spines. Some black and purplish sea-urchins with relatively small bodies and very long spines are dangerous. Their spines can irritate or injure human skin.

Brittle stars sport non-dangerous spines. However, their arms are fragile (which is how they got their name), so it is not advisable to hold or touch them. Through a process known as regeneration, brittle stars are able to regrow lost arms.

Sea cucumbers are not spiny-skinned. They have an ovoid body, similar to the shape of a real cucumber. Sea cucumbers have a habit of shooting out milky and sticky fluid as a defense tactic when disturbed.

Click thumbnail to view full-size

Tridacna maxima | Source

Unidentified mollusk shells from Pixabay | Source

Unidentified land snail from Pixabay | Source

Unidentified squid from Pixabay | Source

Unidentified octopus from Pixabay | Source

Unidentified mollusk shells from Pixabay | Source

5. Phylum Mollusca (Mollusks)

Phylum Mollusca is a group of invertebrate animals whose members can be found both in water and on land. The word Mollusca comes from the Latin term mollis, which means "soft." Because mollusks are soft-bodied animals, most have hard shells made of calcium carbonate to protect their bodies from predators and the environment. Shelled mollusks are divided into two categories: univalves and bivalves. Most also have a muscular ventral foot. There are about 70,000 species of mollusks in the world.

Examples of Mollusca (Mollusks)

Snails are abundant on land and in water and vary in shape, size, color, and design. Snails have muscular ventral feet that they use to move themselves over surfaces.

Squids and octopi are examples of mollusks that don't have outer shells. The skins of these species are more durable than those of shelled mollusks. Squids have ten muscular arms that they use to capture prey.

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Ascaris suum | Source

Ascaris lumbricoides | Source

6. Phylum Nematoda (Nematodes)

Nematoda is a group of invertebrate animals commonly known as nematodes. Nematodes are unsegmented roundworms that are elongated and slender. The word Nematoda comes from the Greek term nematos which means "thread." Nematodes live in a wide variety of environments, including soil, freshwater, saltwater, and in the bodies of plants and animals as parasites.

Examples of Nematoda (Nematodes)

Ascarislumbricoidesis a nematode that often appears in pigs. Ascaris usually live in large numbers in pigs' intestines. They are also known as common roundworms. In humans, they affect children more often than adults. This is thought to be because children are generally less careful than adults in their hygiene habits.

Vinegar eels are not actually eels. They are nematodes whose scientific name is Turbatrix aceti. Generally, vinegar eels are smaller than Ascaris.

Filaria worms, hookworms, and pinworms are all common nematodes that act as parasites in human and animal bodies.

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Araneus diadematus | Source

Gecarcinus ruricola | Source

Cicadella viridis | Source

Androctonus crassicauda | Source

Photo of unidentified centipede from Wikipedia. | Source

Oxidus gracilis | Source

7. Phylum Arthropoda (Arthropods)

Phylum Arthropoda is a group of invertebrate animals with jointed limbs and exoskeletons made of chitin. It is the largest phylum in the animal kingdom.

There are an estimated 750,000 species in this phylum. Of this number, 700,000 species belong to Class Insecta, 25,000 belong to Class Crustacea, 15,000 belong to Class Arachnida, 800 belong to Class Chilopoda, and 200 belong to Class Diplopoda.

8. Phylum Annelida (Annelids)

Phylum Annelida is a group of invertebrate animals with segmented, muscular bodies. The name Annelida comes from the Latin term annulus, which means "ring." This name refers to the ringlike segments of their bodies. The digestive systems of annelids stretch from the mouth to the anus, and different sections of the system have different roles.

Examples of Annelida (Annelids)

Earthworms belong to the phylum Annelida and are abundant in soil. They have cylindrical, segmented bodies. These annelids are beneficial to humans—they help enrich the soil by allowing the air to enter as they burrow through the earth.

Leeches, or Hirudo medicinalis, are another common annelid species. In the past, they have been used to remove blood from individuals for medical purposes.

Marine sandworms, Nereis virens, and Aelosoma are also annelids.

Click thumbnail to view full-size

Nembrotha lineolata | Source

Photo of an unidentified sea squirt from Wikipedia | Source

Polycarpa aurata | Source

Didemnum molle | Source

Polycarpa aurata | Source

Photo of unidentified sea squirts from Wikipedia | Source

Branchiostoma | Source

9. Phylum Chordata (Chordates)

While many species in Phylum Chordata have backbones, there are some that are invertebrates. The name Chordata comes from the Greek word chorde, which means string. While invertebrate chordates don't have backbones, they do have notochord structures that support their bodies.

Examples of Invertebrate Chordates

Sea squirts are animals that squirt water from openings in their body covers when they are touched suddenly. Adult sea squirts live underwater attached to harder objects near beaches. Sea squirts have muscular coats over their bodies that are referred to as "tunics". Members of their subphylum are called tunicates.

Lancelets, or amphioxus, are another type of invertebrate chordate. These slender, fish-like chordates live at the edge of the sea and burrow in the sand.

What is your favorite group of invertebrate animals?

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This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

Questions must be on-topic, written with proper grammar usage, and understandable to a wide audience.

Question:

What are some examples of Sponges?

Answer:

Some examples of Sponges are Calcarea, Hexactinellida, Demospongiae, and Sclerospongiae.

Cephalization is the concentration of sense organs, nervous control, etc., at the anterior end of the body, forming a head and brain, both during evolution and in the course of an embryo's development.

Basically, when we speak of "kinds", it is a level lower than kingdom. So there will be more than 20 or 30 kinds of vertebrate animals. Some of these are spiders, crabs, sponges, insects, and worms. Refer to the Examples category in the first table presented in this article.

There are two major body forms among the Cnidaria - the polyp and the medusa. Sea anemones and corals have the polyp form, while jellyfish are typical medusae. I don't exactly know why there are two forms.

Some of the animal kingdoms which live in shallow oceans are Phylum Arthropoda like crabs and scorpions, Phylum Porifera such as sponges and Phylum Echinodermata like starfish, sea cucumbers, and sea-urchins. On the other hand, the animal kingdoms living in deep oceans are Phylum Mollusca like squids and octopus, Phylum Nematoda like hookworms and pinworms, and Phylum Annelida like earthworms, leeches and lugworms.

Basically all molluscs breathe by gills that are called ctenidia (comb-gills) because of their comb-like shape. In terrestrial molluscs this respiration organ is reduced, but still respiration takes place in the pallial cavity. That is why it is also called the snail's respiratory cavity.

What is the economic importance of these major phyla of invertebrates?

Answer:

Invertebrates are brilliant aerators of soil as well as creating it. In other words, invertebrates not only help us to grow food crops through pollination, they help create and maintain soil quality. This is important for growing in agriculture, as well as in gardens and allotments.

Comments

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Calvin

5 weeks ago

thanks a lot Ray, by any chance could you tell me what your favorite invertebrate is?

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2 months ago

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AUTHOR

Ray

3 months agofrom Philippines

Please leave a comment or suggestions on what topics you would like me to cover about animals. Thanks!

Jamarion Gallaway

4 months ago

i love boys im in love with them

AUTHOR

Ray

4 months agofrom Philippines

Thank you! Goodluck with your homeworks.

Steven

4 months ago

thank you for youer help

billy

4 months ago

good efforts I like it

Brayden

4 months ago

Thank you

AUTHOR

Ray

4 months agofrom Philippines

You're all very welcome. Enjoy reading the article!

ŞÅFĔƏŘÏ ALI

4 months ago

Thanks for your kindness

Ame Amanuel Gadabo

6 months ago

It is very well Note

Danish ali khoso

6 months ago

thanks for given me knowledge

IM YOU IN THE FUTURE

7 months ago

hi thanks for the help also pls answer my question thanks:]

Viky

7 months ago

Thank you very much for your help

Science

8 months ago

This help me with my homework thanks...... Tomorrow I will be active in school.....

Miebakagh Fiberesima

12 months agofrom Port Harcourt, Rivers State, NIGERIA.

I can only note significant comments that relate to and is relevant to the article. All writers commenting would kindly take that into consideration. Many thanks, and enjoy the weekend.

lati

12 months ago

hi rayguy

Someone

13 months ago

This helped me with my science homework thank you so much

Miebakagh Fiberesima

13 months agofrom Port Harcourt, Rivers State, NIGERIA.

All new comments were well noted. Thanks.

Abbas Ali

13 months ago

Good Job Sir

AUTHOR

Ray

14 months agofrom Philippines

Thank you very much Linus Divine. I am glad you enjoyed reading my article. You can share it with your friends and relative s who are interested with invertebrate animals as well. Have a great day!

Linus Divine

14 months ago

I enjoy this website a lot for its simplest explanation of terms, thank you so much.

Miebakagh Fiberesima

14 months agofrom Port Harcourt, Rivers State, NIGERIA.

Hi. I agreed. Thanks.

anwaarnbatala@gmail.com

14 months ago

thank you very much this website made me learn a lot thank you very much

Lol

17 months ago

Ew yak

Miebakagh Fiberesima

22 months agofrom Port Harcourt, Rivers State, NIGERIA.

Hello JR, thanks for the feedback. I even like science subjects that related to the living organism. Will be watching for your articles.

AUTHOR

Ray

23 months agofrom Philippines

Me too, Sir Eric. I love eating mollusks. We have here in the Philippines a famous dish called "Adobong Pusit" (Squid Ink), and it tastes so good.

Thanks, Sir Eric. Although I don't know if the pictures are children-friendly. Some pictures are cringy because of the slimy appearance. But, I hope you and your child enjoy reading and learning from my article. Have a great day!

Eric Dierker

23 months agofrom Spring Valley, CA. U.S.A.

Mollusks because they are fun to eat!

Well done. A much needed refresher as my son and I are way into knowing this stuff.

AUTHOR

Ray

23 months agofrom Philippines

Thanks, Miebakagh. I also love Biology eventhough it is not included in my path of study anymore. I was once a school representative in different Science competitions in our city so I was so inlove with this subject.

Expect for more Biology-related articles Miebakagh. I'll be writing more science articles that I love. Thank you so much for the interest. Have a great day!

Miebakagh Fiberesima

23 months agofrom Port Harcourt, Rivers State, NIGERIA.

Hi John, thanks for the update. I read Biology in secondary grammar school. It is one of my favorite and best science subject, and I made "A" in my external exams. I consistently it biology and related subjects and apply them in my life.

AS an avid reader, I would read any subject that relate Biology. I will be watching for your fourth coming articles always.

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